A number of protein neurotrophic factors, or neurotrophins, have been identified which influence growth and development of the vertebrate nervous system. It is believed that these factors play an important role in promoting the differentiation, survival, and function of diverse groups of neurons in the brain and periphery.
The belief that neurotrophic factors have important signalling functions in neural tissues is based upon the precedent established by work with nerve growth factor (NGF). NGF has been shown, both in vitro and in vivo, to support the survival of sympathetic, sensory, and basal forebrain neurons. Administration of exogenous NGF rescues neurons from cell death during development. Conversely, removal or sequestration of endogenous NGF by administration of anti-NGF antibodies promotes such cell death. Heumann, J. Exp. Biol. 132:133-150 (1987); Hefti, J. Neurosci. 6:2155-2162 (1986); Thoenen and Barde, Annu. Rev. Physiol. 60:284-335 (1980).
Additional neurotrophic factors related to NGF have since been identified. These include brain-derived neurotrophic factor (BDNF) (Leibrock, et al., Nature 341:149-152 (1989)), neurotrophin-3 (NT-3) (Kaisho, et al., FEBS Lett. 266:187 (1990); Maisonpierre, et al., Science 247:1446 (1990); Rosenthal, et al., Neuron 4:767 (1990)), and neurotrophin 4/5 (NT-4/5) (Berkmeier, et al., Neuron 7:857-866 (1991)).
Neurotrophins, similar to other polypeptide growth factors, affect their target cells through interactions with cell surface receptors. According to our current understanding, two kinds of transmembrane glycoproteins act as receptors for the known neurotrophins. Equilibrium binding studies have shown that neurotrophin-responsive neuronal cells possess a common low molecular weight (65,000-80,000 Daltons), low affinity receptor, typically referred to as p75.sup.LNGFR or p75, and high molecular weight (130,000-150,000 Daltons), high and low affinity receptors that are members of the trk family of receptor tyrosine kinases.
Receptor tyrosine kinases are known to serve as receptors for a variety of protein factors that promote cellular proliferation, differentiation, and survival. In addition to the trk receptors, examples of other receptor tyrosine kinases include the receptors for epidermal growth factor (EGF), fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF). Typically, these receptors span the cell membrane, with one portion of the receptor being intracellular and in contact with the cytoplasm, and another portion of the receptor being extracellular. Binding of a ligand to the extracellular portion of the receptor is believed to induce tyrosine kinase activity in the intracellular portion of the receptor, with ensuing phosphorylation of various intracellular proteins involved in cellular signalling pathways.
In addition to receptor tyrosine kinases that serve as receptors for known protein factors, many receptor-like tyrosine kinases have been identified for which no ligand is known. Examples of such "orphan" receptors include recently discovered members of the eph family, which includes eph, elk, cek5, cek7, mek4/cek4/hek, sek, hek2, and bsk (Tuzi, et al., Br. J. Cancer 69:417-421 (1994); Zhou, et al., J. Neurosci. Res. 37:129-143 (1994)). Recently, a protein ligand has been identified for another eph family member, eck (Bartley, et al., Nature 368:558-560 (1994)).
Although eph family members are expressed in many different tissues, several family members are expressed in the nervous system or specifically in neurons (Maisonpierre, et al., Oncogene 8:3277-3288 (1993); Lai, et al., Neuron 6:691-704 (1991). In order to better understand the role of these and other orphan receptor tyrosine kinases in the nervous system, it would be useful to identify new ligands that bind to such receptors.
The present invention is based on successful research resulting in the identification, cloning, and sequencing of an eph-related tyrosine kinase receptor, referred to as REK7, and its ligand, referred to as AL-1.
It is an object of the present invention to provide nucleic acid encoding AL-1, and to use the nucleic acid to produce AL-1 in recombinant host cells for diagnostic use or for therapeutic use.
It is another object to use such nucleic acids encoding AL-1, and portions thereof, to identify related nucleic acids in the cells or tissues of various animal species.
It is another object to provide derivatives and modified forms of AL-1, including amino acid sequence variants and covalent derivatives thereof, as well as antagonists of AL-1. It is another object to prepare immunogens for raising antibodies, as well as to obtain antibodies, capable of binding to AL-1, or derivatives or modified forms thereof.
These and other objects of the invention will be apparent to the ordinary artisan upon consideration of the specification as a whole.